Mesozoic Mammal Colors

Arboroharamiyia fuscus by Chuang Zhao and Rushoang Li

A recent study covered six Mesozoic mammaliaform fossils and found them to be predominantly dark brown or black, though they covered groups as far apart as haramiyidans and eutriconodonts.

The explanation given was likely due to nocturnality. Several previous studies suggested a primarily nocturnal lifestyle for Mesozoic mammals regardless of their ecological diversity, so it checks out. After all, many modern mammals are still nocturnal, from bats to Javan rhinos.

This will have little impact on my speculative evolution project, Multituberculate Earth, since like in our timeline mammals expanded into all sorts of niches after the extinction of non-avian dinosaurs. Some are even brightly colourful like mandrills.